U.S. Pat. Nos. 4,533,574 and 4,562,100 both show polyimide coating compositions from esterified anhydrides and aromatic amine mixtures. Both patents, however, (see col. 3, lines 43-55) specifically say that for their purposes, aromatic dianhydrides, such as pyromellitic dianhydride, are not generally suitable. The patents both teach that it is desirable that the diamines have amine groups in the meta position relative to each other (see col. 2, line 20).
U.S. Pat. No. 2,880,230 is concerned with diamine salts of pyromellitic acid diester. The patent teaches at col. 2, lines 54-59 that in the preparation of salts to be used for the preparation of polyimides, the mixture of acid diester isomers may be used as the acidic component or that either isomer may be isolated from the mixture and used alone, but that there is no particular advantage in so doing if polymer-grade intermediates are used.